This invention relates to a high sensitivity conductivity detector, and more particularly, to a detector which can be used in ion chromatography.
Recently, there has been a growing need for reliable techniques for determining mixtures of ions in diluted solutions of solvent. ANION CHROMATOGRAPHY WITH LOW-CONDUCTIVITY ELUENTS by D. T. Gjerde, J. S. Fritz and G. Schmuckler, J. of Chromatography, 186 (1979) pp 509-519 describes the ion chromatography (I.C.) technique. Typically, ion chromatography uses a separation column containing an anion exchange resin. The separated anions are detected after passing through a suppressor column to remove background conductance. The disadvantages of use of a suppressor column have been recognized.
Recently, an attempt has been made to perform ion chromatography measurements in a system which does not have a suppressor column.
"ION CHROMATOGRAPHY WITH CONVENTIONAL HPLC INSTRUMENTATION", by J. E. Girard and J. E. Glatz, American Laboratory, October 1981, pp. 26-35 describes a non-suppressed ion chromatography system which includes a Wescan model 213 conductivity detector.
The detection of conductivity changes in such a system is extremely difficult. Typically, a change in conductivity of one part in 30,000 must be measured. Prior art conductivity detectors, such as the CONDUCTOMONITOR.TM. supplied by Milton Roy Company, have been successfully used in other chromatography measuring techniques, but are not suitable for use in ion chromatography.
One requirement for making measurements of extremely small conductivity changes, is that the temperature of the sample be constant. In other chromatography applications, provision is made for compensating the measurement for temperature changes. The desirability of maintaining the temperature of the sample constant has been recognized, for example, in the aforementioned Wescan instrument in which the sample is heated to a temperature above ambient temperature and maintained at that temperature by control of the heater.
It is an object of the present invention to provide a conductivity detector in which the sample is accurately maintained at a temperature which is approximately ambient temperature, thereby obviating the need for large heat transfers to the sample.
It is another object of the present invention to replace the Wheatstone bridge measurement technique of prior art conductivity detectors with a measurement circuit which is capable of measuring very small changes in conductivity.
It is another object of the present invention to provide a measurement circuit in which a small excitation voltage is applied to the sample cell.